The present invention relates generally to the field of oscillators and more particularly to voltage controlled oscillators of the type used, for example, in phase locked loop (PLL) circuits.
Voltage controlled oscillator (VCO) circuits produce output signals whose frequency varies as a function of an input control voltage. These circuits are widely used in systems employing PLL circuits as well as in any system where a variable frequency output signal is required as a function in an input signal.
In data communication systems PLL circuits have been employed for extracting clock information from a received signal which permits data processing systems at remote locations to operate synchronously. Examples of such applications are described in U.S. Pat. No. 4,131,861 to Malaviya entitled "Variable Frequency Oscillator System Including Two Matched Oscillators Controlled by a Phase Locked Loop," and U.S. Pat. No. 4,513,427 to Boriello et al. entitled "Data and Clock Recovery System for Data Communication Controller." VCOs in these applications preferably have a highly stable center frequency at or near the desired system clock frequency, and a high gain or change in output frequency for a given change in input voltage. Finally, it is particularly advantageous if the VCO can be integrated on the same chip with the PLL and other circuits in order to reduce the size, power requirements, delay and cost of the system.
Highly stable VCOs have been fabricated using inherently stable frequency-determining components such as varactor diodes, bulk crystal devices, and bulk and surface acoustic wave devices. A relatively high VCO gain (MHz/volt) can be achieved by means of operational amplifiers such as in U.S. Pat. No. 4,118,674, which employs a crystal oscillator. However, these components are not integratable on a chip with the other required components in a VCO or PLL.
Another VCO configuration used in prior systems employs cascaded inverters connected in a ring configuration to form a ring oscillator as described, for example, in U.S. Pat. No. 4,072,910 to Dingwall et al. entitled "Voltage Controlled Oscillator Having Equally Controlled Current Source and Current Sink." Other ring oscillator VCOs are described in U.S. Pat. Nos. 4,052,673 and 4,513,427. Such oscillators have the advantages of small size, low cost, a large tuning range and they are integratable. The maximum frequency of a ring oscillator is limited by the logic speed, i.e., the switching delay of the devices used to form the inverter circuits. U.S. Pat. No. 4,513,427, cited above, describes a ring oscillator VCO employing a tapped delay line. While the components are contained on a single integrated circuit chip, the oscillator frequency in the disclosed embodiment is on the order of 40 MHz. and the data transfer rate is 10 Mbits/sec. For much higher data rates, which necessitate even higher clock frequencies, the tapped delay line technique will not suffice.
In the case of data processing systems including communication links wherein the data transfer rates are on the order of several hundred Mbits/sec., a VCO having a center frequency of several hundred MHz. and a gain of at least 1 GHz./volt is required in order to achieve acceptable noise performance. Known VCO arrangements cannot achieve this performance with a circuit formed on a single integrated circuit chip.